Intermittent cw system for satellite surveillance

ABSTRACT

An electronic satellite surveillance system wherein transmitters and receivers are located at the same sites and wherein the transmitted signals consist of multifrequency, intermittent, CW electromagnetic energy which is radiated in narrow fan patterns and wherein the receivers include means which allow unambiguous angle and range determination.

United States Patent 1 1 3,725,924 Easton 1 Apr. 3, B973 54]INTERMITTENT CW SYSTEM FOR 3,271,763 9/1966 Colby, Jr. ..343/12 ASATELLITE SURVEILLANCE [75] Inventor: Roger L. Easton, Oxon Hill, Md.Primary ExammeFBen-lami Borchelt Assistant Examiner-G. E. Montone [73]Asslgnee: The Umed States of Amenca as Attorney-R. S. Sciascia, ArthurL. Branning and represented by the Secretary of the James G Murray Navy[22] Filed: July 22, 1970 [57] ABSTRACT [21] Appl. N -I 56,03 Anelectronic satellite surveillance system wherein transmitters andreceivers are located at the same sites 52 0.5. Ci. ..343/10, 343/100 STand wherein the transmitted Signals Consist Of [51] Int. Cl ..G01s 9/04tifrequeney, intermittent, CW electromagnetic n rgy [58] Field of Search..343/l00 ST, l0, 12 A, 7.5, which is radiated in narrow fan patternsand wherein 343/5 PD the receivers include means which allow unambiguousangle and range determination. [56] References Cited 8 Claims, 4 DrawingFigures UNITED STATES PATENTS 3,122,741 2/1964 Easton ..343/100 STPATEHTEQAFM I373 SHEET 1 OF 2 F/G. f

PRIOR ART FIG. 2

PRIOR ART INVENTOR ROGER 1.. EAST 0N BY K' W 1 METTORNEE PATEHHZQAFRE1375 3,725,924

SHEET 2 BF 2 INVENTOR ROGER L. EAS TON ATTORNEY5 INTERMITTENT CW SYSTEMFOR SATELLITE SURVEILLANCE STATEMENT OF GOVERNMENT INTEREST BACKGROUNDOF THE INVENTION In the past decade, due to advances in spacetechnology, there has been a tremendous and ever increasing number ofman-made satellites .placed in orbit around the earth. For many obviousreasons it became desirable for the government to develop systems whichwould detect and monitor the orbiting satellites that pass over thecontinental United States. One of these systems is the Navy SpaceSurveillance System which is described in US. Pat. No. 3,122,741 issuedto Roger L. Easton on Feb. 25, 1964 and the related U.S. Pat. No.3,125,756 issued to Maxime G. Kaufman and Leonard 0. Hayden on Mar. 19,1964. US. Pat. No. 3,504,367 issued on Mar. 31, 1970 to Roger L. Eastondescribes a frequency modulation technique which allows the unambiguousdetermination of the range of the satellite. As described more fully inthese patents, the Navy Space Surveillance System includes transmitterswhich erect a fence or thin wall of continuous CW energy across thesouthern portion of the continental United States. Orbiting satellites,upon crossing through this fence, reflect a portion of the CW energy toreceiver sites where radio interferometer techniques, implemented by theappropriately spaced antennas and phase measuring apparatus, provide anunambiguous indication of the direction and range of the satellite fromthe receiver site. To avoid direct coupling, the transmitter andreceiver sites are separated by hundreds ofmiles, a circumstance whichgreatly increases the land acquisition, construction and maintenancecosts.

SUMMARY OFTHE INVENTION The invention here disclosed and described is animprovement on the prior systems for detecting and monitoring orbitingsatellites, and of the Navy Space Surveillance System in particular, andwhile embracing all of the advantages of the prior systems provides fora large reduction in those costs which relate to site acquisition,construction and-maintenance. To attain this advantageous costreduction, the invention contemplates an electronic system of satellitesurveillance in which transmitters and receivers are located at the samesites, thereby reducing the number of necessary sites and obviouslyproducing a sizable reduction in the cost of site acquisition,construction and maintenance. To avoid direct coupling between thetransmitter and receiver, the invention contemplates the intermittentradiation of multi-frequency energy in a narrow fan or fence" patternwith reception occurring during the transmitter off period. As in priorsystems, the receivers include a plurality of antennas and phasemeasuring apparatus which allows the unambiguous determination of thedirection and range of the satellite.

OBJECTS OF THE INVENTION It is, therefore, an object of the invention toprovide an improved satellite surveillance system.

Another object is to reduce the cost of satellite surveillance systemsby providing an electronic system for satellite surveillance whereintransmitters and receivers are located at the same sites, therebyreducing the cost of site acquisition, construction and maintenance.

Yet another object is to provide an improved satellite surveillancesystem wherein multi-frequency, intermittent, CW energy is radiated innarrow fan patterns and the receivers include phase measuring apparatusand a plurality of antennas which allow unambiguous range and angledetermination.

DESCRIPTION OF THE DRAWINGS The exact nature of this invention as wellas other objects and advantages thereof will be readily apparent fromconsideration of the following specification relating to the annexeddrawing in which:

FIGS. 1 and 2 show the priorart and FIGS. 3 and 4 illustrate theimprovement of the prior art as embodied in this invention. I

DESCRIPTION OF THE INVENTION Referring now to the drawings, whereinlike.

reference numerals and letters designate like or corresponding partsthroughout the several views. The reader is cautioned that the drawingsshould not be construed as being more than merely representative ofpractical structure. Obviously much of the drawings and many of thefeatures of the invention have been dimensionally distorted,symbolically shown and otherwise extremely simplified for the purposethat the disclosure of the invention might be presented in a ferometertechniques which are employed) and phase and frequency measuringdevices, which can either be at the site or at remote computing centers.

Transmitters I8, 20 and 22 are alternately located with the receivers10, l2, l4 and 16, all of which are substantially on a straight line. Toavoid direct coupling to the receiving stations, the transmitters 18,20and 22 are separated from the receiving stations by, at least, severalhundred miles. The transmitters 18, 20 and 22 cooperatively function toerect a thin fence" of CW electromagnetic energy, this fence 24 beingformed by the individual fan shaped radiation patterns of thetransmitters, such as the pattern 26 of transmitter 20 which is shown inFIG. 2. It has been found that improved results are obtainable if the CWfence 24 includes a plurality of frequencies which are coherentlyrelated.

The thickness of fence 24 and the speed of satellite S (shown in a polarNorth-South orbit) are such that satellite S is typically in the fencefor only a relatively short period of time, i.e., approximately asecond. During this approximately one second interval, the CWtransmission from transmitter is reflected from satellite S to receivingstations 12 and 14 where well known techniques are employed to obtaininformation concerning the orbital parameters of the satellite S. Forexample, interferometer techniques are employed to obtain intersectingvectors from sites 12 and 14 which define the location of satellite S asit crossed the fence 24.

The reader is now referred to FIGS. 3 and 4 which illustrate the presentinvention as located in the environment of FIGS. 1 and 2, althoughobviously the invention is not limited to such a location. In contrastto the seven sites of the prior system of FIG. 1, the present inventionrequires only the three sites 30, 32 and 34 which are shown in FIG. 3 asbeing located on a straight line across the southern continental UnitedStates. Each of the sites 30, 32 and 34 include both trans- I mittingand receiving equipments which are controlled radiate fan shapedpatterns 26A which are (in some ways) very similar to the patterns 26which are radiated by the transmitters 18, 20 and 22 of FIG. I in thatthe patterns 26A include a plurality of frequencies which are coherentlyrelated and cooperatively function to erect a thin fence" 24A ofelectromagnetic energy across the southern portion of the continentalUnited States. However, in contrast to the continuous CW energy of theprior art fence 24 and patterns 26 shown in FIG. 1, the fence 24A andthe individual patterns 26A are intermittent CW with a typical dutycycle of 50 percent, i.e., on and off" periods of equal duration.

The on period of fence 26A and, of course, the

transmitters at the sites 30, 32 and 34, is preferably related to theround trip time of the radiation (signal) to the maximum desired rangeof the system, typically an on period of approximately 10 millisecondsfor a desired maximum range of 1000 miles. If the satellite Stransverses the fence 24A at the maximum range of 1000 miles, the full10 millisecond signal reflected by satellite S will be processed by thereceiver at the site,

i.e., site 32 as illustrated in FIG. 4. When satellite S is at a shorterrange, only a latter portion of the reflected signal will be processedsince the unprocessed earlier portion of the reflected signal willarrive while transmission is occurring and the receiving equipment isturned off. In other words, the length of the processed signal isindicative of the range of satellite S, the shorter the processed signalthe shorter the distance to the satellite. The usual interferometer,doppler, phase measuring and other techniques are also applicable forobtaining the orbital parameters of the satellite S.

It will, of course, be apparent to those skilled in the electronic artsthat other techniques of intermittent transmission can also be used. Forexample, the transmitters at sites 30, 32 and 34 may be operated with 50percent duty cycles and on-off periodsin sequences of l, 2, 3 etc.milliseconds. Because of interpretive difficulties these othertechniques have not been found to be as desirable as the previouslydescribed 10 millisecond pulse technique.

By now the apparatus and operation of the present invention is, nodoubt, evident to the reader. As contemplated, the invention embodies asatellite surveillance system located at one or more sites, each siteincluding transmitting and receiving apparatus. As shown in FIG. 3,three such sites 30, 32 and 34 comprise a system for surveillance ofsatellites passing over the continental United States. The transmittingand receiving apparatuses at the sites are operatively coordinated tointermittently erect (in a direction perpendicular to the expected pathsof satellites) a thin fence of CW electromagnetic energy which includesfrequency components that are coherently related and to intermittentlybe capable of receiving energy reflected from a satellite 5 passingthrough the fence. Typically the transmitting and receiving apparatusesat all of the sites operate together on 50 percent duty cycles, i.e.,all of the transmitters radiating fan shaped patterns 26A to fonn thefence 24A, typically for a period of 10 milliseconds, during which timethe receiving apparatuses at the sites are inhibited from processing theenergy being radiated. The radiated energy is, of course, quite stronglydirectly coupled into the antennas of the receiving equipments. When thetransmitting equipments are turned off, the receiving equipments at thevarious sites are made operative to process the energy reflected by asatellite S passing through the fence 24A. This processing iscontemplated as including interferometer techniques which utilizeappropriately spaced antennas to determine the direction of thesatellite and modulations ranging techniques as disclosed in US. Pat.No. 3,504,367 issued on Mar. 31, I970 to Roger L. Easton. The distance,of the satellite S is roughly determined by by measuring the duration ofthe signal processed, i.e., the full 10 millisecond signal is processedwhen the satellites is at the maximum system range of approximately 1000miles while smaller portions of the signal are processed when thesatellite S is at a lesser distance as it passes through the fence 24A.Precise ranging is obtained by the modulation techniques of thementioned patent. It is also contemplated that other known techniquesare used to determine other orbital parameters of the satellite S.

There has been disclosed an improved satellite surveillance. systemwherein transmitters and receivers are located at the same sites,thereby reducing the cost of site acquisition. construction andmaintenance and wherein multi-frequency, intermittent, CW energy isradiated in narrow fan patterns and the receivers include phasemeasuring apparatus and a plurality of antennas which allow unambiguousrange and angle determinations. Obviously many modifications andvariations of the present invention are possible in the light of theabove teachings. It is therefore to be understood, that within the scopeof the appended claims, the invention may be practiced otherwise than asspecifically described. I

What is claimed and desired to be secured by Letters Patent of theUnited States is:

1. Improved apparatus for detecting satellites in orbit around theearth, comprising:

transmitting means which operate on substantially a 50 percent dutycycle and function during the on portion of said cycle to radiatecoherent multifrequency intermittent continuous wave electromagneticenergy in a pattern having dimensions which are relatively very small inthe expected direction of the path of said satellite and relatively verylarge in the directions perpendicular to said path and receiving meanswhich are operative only during the of portion of said cycle forreceiving energy reflected by a satellite passing through said radiatedpattern and for determining from said reflected energy the range, inaccordance with the duration of the reflected wave energy receivedduring the off portion of the cycle, and direction of said satellite. 2.The apparatus of claim 1 wherein said transmitting means and saidreceiving means are located at the same site.

3. The apparatus of claim 1 wherein the duration of a plurality of siteslocated on a line that is substantially straight, each site includingtransmitting means and receiving means;

each of said plurality of transmitting means operating on approximately50 percent duty cycles which are substantially coincident andcooperatively functioning during the on portion of said duty cycle toradiate a fence of coherent multifrequency intermittent continuous waveelectromagnetic energy which is substantially parallel to said line andwhich has dimensions that are relatively very small in the expecteddirection of the path of said satellite and relatively very large in thedirection perpendicular to said path and each of said plurality ofreceiving means operating only during the off portion of said duty cyclefor receiving energy reflected by a satellite passing through said fenceand for determining from said reflected energy the range, in accordancewith the duration of the reflected wave energy received during the offportion of the cycle, and direction of said satellite.

6. The system of claim 5 wherein the duration of said on portion of saidduty cycle is approximately equal to the round trip time ofelectromagnetic energy to the maximum satellite detection range of saidsystem.

7. The system of claim 6 wherein said receiving means utilizeinterferometer techniques to determine the direction of said satellite.

8. The system of claim 7 wherein the energy radiated by saidtransmitting means includes different frequency components which arecoherently related.

1. Improved apparatus for detecting satellites in orbit around theearth, comprising: transmitting means which operate on substantially a50 percent duty cycle and function during the ''''on'''' portion of saidcycle to radiate coherent multifrequency intermittent continuous waveelectromagnetic energy in a pattern having dimensions which arerelatively very small in the expected direction of the path of saidsatellite and relatively very large in the directions perpendicular tosaid path and receiving means which are operative only during the''''off'''' portion of said cycle for receiving energy reflected by asatellite passing through said radiated pattern and for determining fromsaid reflected energy the range, in accordance with the duration of thereflected wave energy received during the ''''off'''' portion of thecycle, and direction of said satellite.
 2. The apparatus of claim 1wherein said transmitting means and said receiving means are located atthe same site.
 3. The apparatus of claim 1 wherein the duration of said''''on'''' portion of said transmitting means 50 percent duty cycle isapproximately equal to the round trip time of electromagnetic energy tothe maximum range at which said improved apparatus can detectsatellites.
 4. The apparatus of claim 3 wherein said transmitting meansand said receiving means are located at the same site.
 5. An improvedelectronic system for detecting satellites in orbit around the earth,comprising: a plurality of sites located on a line that is substantiallystraight, each site including transmitting means and receiving means;each of said plurality of transmittinG means operating on approximately50 percent duty cycles which are substantially coincident andcooperatively functioning during the ''''on'''' portion of said dutycycle to radiate a fence of coherent multifrequency intermittentcontinuous wave electromagnetic energy which is substantially parallelto said line and which has dimensions that are relatively very small inthe expected direction of the path of said satellite and relatively verylarge in the direction perpendicular to said path and each of saidplurality of receiving means operating only during the ''''off''''portion of said duty cycle for receiving energy reflected by a satellitepassing through said fence and for determining from said reflectedenergy the range, in accordance with the duration of the reflected waveenergy received during the ''''off'''' portion of the cycle, anddirection of said satellite.
 6. The system of claim 5 wherein theduration of said ''''on'''' portion of said duty cycle is approximatelyequal to the round trip time of electromagnetic energy to the maximumsatellite detection range of said system.
 7. The system of claim 6wherein said receiving means utilize interferometer techniques todetermine the direction of said satellite.
 8. The system of claim 7wherein the energy radiated by said transmitting means includesdifferent frequency components which are coherently related.